Comments on: Extended-term effects of head and neck irradiation in a rodent. Eur J Cancer 2001, 37, 1938-1945

Prevention and treatment of radiotherapy-induced side effects

Radiotherapy remains a mainstay of cancer treatment, being used in roughly 50% of patients. The precision with which the radiation dose can be delivered is rapidly improving. This precision allows the more accurate targeting of radiation dose to the tumor and reduces the amount of surrounding normal tissue exposed. Although this often reduces the unwanted side effects of radiotherapy, we still need to further improve patients' quality of life and to escalate radiation doses to tumors when necessary. High-precision radiotherapy forces one to choose which organ or functional organ substructures should be spared. To be able to make such choices, we urgently need to better understand the molecular and physiological mechanisms of normal tissue responses to radiotherapy. Currently, oversimplified approaches using constraints on mean doses, and irradiated volumes of normal tissues are used to plan treatments with minimized risk of radiation side effects. In this review, we discuss the responses of three different normal tissues to radiotherapy: the salivary glands, cardiopulmonary system, and brain. We show that although they may share very similar local cellular processes, they respond very differently through organ-specific, nonlocal mechanisms. We also discuss how a better knowledge of these mechanisms can be used to treat or to prevent the effects of radiotherapy on normal tissue and to optimize radiotherapy delivery.

Radioprotective effect of amifostine on parotid gland functioning is region dependent

PURPOSE

To investigation the protective ability of amifostine during partial irradiation of the rat parotid gland.

METHODS AND MATERIALS

Single-dose X-ray irradiation was performed by use of collimators with conformal radiation portals for either the 100% volume (15 Gy) or the 50% cranial/caudal partial parotid gland volumes (30 Gy). Amifostine was administered intraperitoneally at a dose of 250 mg per kg body weight, 25 minutes before irradiation. Saliva flow rates, gland weights, and the tissues of the individual lobes were investigated up to 1 year after treatment.

RESULTS

A clear protective effect of amifostine was found against loss of saliva flow, the altered appearance of gross morphology, loss of gland weight, and histopathologic changes for the 100% volume gland irradiations and for the 50% volume cranial irradiations but not for the 50% volume caudal irradiations.

CONCLUSIONS

The protective ability of amifostine is strongly dependent on the irradiated glandular region and observed for later damage only. The major effect of the drug seems to be the prevention of volume effects caused by secondary damage that occurs in shielded parts of the gland. The results of the present study show that understanding of the anatomy and physiology of organs that are to be spared is necessary to ensure optimal preservation of function.

Secondary radiation damage as the main cause for unexpected volume effects: a histopathologic study of the parotid gland

PURPOSE

To elucidate with a histopathological study the mechanism of region-dependent volume effects in the partly irradiated parotid gland of the rat.

METHODS AND MATERIALS

Wistar rats were locally X-irradiated with collimators with conformal radiation portals for 100% volume and 50% cranial/caudal partial volumes. Single doses up to 40 Gy were applied. Parotid saliva samples were collected, and the three lobes of the parotid gland were examined individually on the macro- and micromorphologic level up to 1 year after irradiation.

RESULTS

Dose-dependent loss of gland weight was observed 1 year after total or partial X-irradiation. Weight loss of the glands correlated very well with loss of secretory function. Irradiating the cranial 50% volume (implicating a shielded lateral lobe) resulted in substantially more damage in terms of weight loss and loss of secretory function than 50% caudal irradiation (shielding the ventral and dorsal lobe). Histologic examinations of the glands 1 year after irradiation revealed that the shielded lateral lobe was severely affected, in contrast to the shielded ventral and dorsal lobes. Time studies showed that irradiation of the cranial 50% volume caused late development of secondary damage in the shielded lateral lobe, becoming manifest between 240 and 360 days after irradiation. The possible clinical significance of this finding is discussed.

CONCLUSION

It is concluded that the observed region-dependent volume effect for late function loss in the rat parotid gland after partial irradiation is mainly caused by secondary events in the shielded lateral lobe. The most probable first step (primary radiation event) in the development of this secondary damage is radiation exposure to the hilus region (located between the ventral and dorsal lobe). By injuring major excretory ducts and supply routes for blood and nerves in this area, the facility system necessary for proper functioning of the nonexposed lateral lobe is seriously affected. The unexpected volume effect in the rat might have consequences for treatment strategies in radiotherapy, implicating not only salivary glands but also other organs with a seemingly homogeneous distribution of radiosensitive elements, a situation wherein volume effects have not been anticipated up to now.

On the mechanism of salivary gland radiosensitivity

PURPOSE

To contribute to the understanding of the enigmatic radiosensitivity of the salivary glands by analysis of appropriate literature, especially with respect to mechanisms of action of early radiation damage, and to supply information on the possibilities of amelioration of radiation damage to the salivary glands after radiotherapy of head-and-neck cancer.

METHODS AND MATERIALS

Selected published data on the mechanism of salivary gland radiosensitivity and radioprotection were studied and analyzed.

RESULTS

From a classical point of view, the salivary glands should not respond as rapidly to radiation as they appear to do. Next to the suggestion of massive apoptosis, the leakage of granules and subsequent lysis of acinar cells was suggested to be responsible for the acute radiation-induced function loss of the salivary glands. The main problem with these hypotheses is that recently performed assays show no cell loss during the first days after irradiation, while saliva flow is dramatically diminished. The water secretion is selectively hampered during the first days after single-dose irradiation. Literature is discussed that shows that the compromised cells suffer selective radiation damage to the plasma membrane, disturbing signal transduction primarily affecting watery secretion. Although the cellular composition of the submandibular gland and the parotid gland are different, the damage response is very alike. The acute radiation-induced function loss in both salivary glands can be ameliorated by prophylactic treatment with specific receptor agonists.

CONCLUSIONS

The most probable mechanism of action, explaining the enigmatic high radiosensitivity for early effects, is selective radiation damage to the plasma membrane of the secretory cells, disturbing muscarinic receptor stimulated watery secretion. Later damage is mainly due to classical mitotic cell death of progenitor cells, leading to a hampered replacement capacity of the gland for secretory cells, but is also caused by damage to the extracellular environment, preventing proper cell functioning.

Volume effects and region-dependent radiosensitivity of the parotid gland

PURPOSE

To detect volume effects and possible regional differences in radiosensitivity of the rat parotid gland.

METHODS AND MATERIALS

Parotid glands of male albino Wistar rats were locally X-irradiated, with collimators with conformal radiation portals used to supply 100% volume and 50% cranial/caudal partial volumes. High-resolution magnetic resonance imaging was used to provide the outlines of the parotid glands. Single doses of up to 40 Gy were applied, and the effects on saliva secretion, measured with the aid of miniaturized Lashley cups, were followed up to 365 days after the irradiation.

RESULTS

Under conditions of equal mean absorbed doses and small variations in dose distribution, a pertinent volume effect was observed for late but not for early radiation damage. The late effects were different for the cranial part as compared with the caudal part of the parotid gland. The reduction in flow rate was much more severe after irradiation in the cranial part. After a single dose of 30 Gy, the reductions in flow rates were approximately 65% and 25% for the cranial and caudal parts, respectively. At that dose, no saliva flow was observed after irradiation of 100% of the gland.

CONCLUSION

From the rat model studies presented, it is concluded that late radiation damage after partial irradiation of parotid glands shows region-dependent volume effects. This finding is expected to be relevant to the radiosensitivity of human salivary glands, and it implies that the predictive power of the mean dose concept in radiotherapeutic practice is limited. The finding of region-dependent late radiation damage also challenges the basic assumptions of most current normal tissue complication probability models for parotid gland function.

Oral sequelae of head and neck radiotherapy

In addition to anti-tumor effects, ionizing radiation causes damage in normal tissues located in the radiation portals. Oral complications of radiotherapy in the head and neck region are the result of the deleterious effects of radiation on, e.g., salivary glands, oral mucosa, bone, dentition, masticatory musculature, and temporomandibular joints. The clinical consequences of radiotherapy include mucositis, hyposalivation, taste loss, osteoradionecrosis, radiation caries, and trismus. Mucositis and taste loss are reversible consequences that usually subside early post-irradiation, while hyposalivation is normally irreversible. Furthermore, the risk of developing radiation caries and osteoradionecrosis is a life-long threat. All these consequences form a heavy burden for the patients and have a tremendous impact on their quality of life during and after radiotherapy. In this review, the radiation-induced changes in healthy oral tissues and the resulting clinical consequences are discussed.